Novel insight into the mesoscale texture of cement hydrates

Research by ISM member Prof. Del Gado, which appeared this month on PNAS early edition, has provided new understanding of cement hydrates' mesoscale texture, a crucial step to connect the fundamental scales to the macroscale of engineering properties in cement and concrete. Cement production for concrete is responsible of 5-10% of the world man-made CO2 and new fundamental understanding is needed to develop green formulations while mantaining the mechanical performance of the material. With her collaborators at MIT CSHub, University Pierre et Marie Curie, Aix-Marseille University, University of Newcastle and UCLA, Del Gado developed a computational approach that combines information of the nanoscale building units of C–S–H and their effective interactions, obtained from atomistic simulations and experiments, into a statistical physics framework for aggregating nanoparticles.

Upon dissolution of cement powder in water, calcium–silicate–hydrates (C–S–H) precipitate and assemble into a cohesive gel that fills the pore space in the cement paste over hundreds of nanometers and binds the different components of concrete together. The mechanics and microstructure are key to concrete performance and durability, but the level of understanding needed to design new, more performant cements and have an impact on the CO2 footprint of the material is far from being reached.

Controlling structure and properties of C–S–H is a challenge, due to the complexity of this hydration product and of the mechanisms that drive its precipitation from the ionic solution upon dissolution of cement grains in water. Departing from traditional models mostly focused on length scales above the micrometer, recent research addressed the molecular structure of C–S–H. However, small-angle neutron scattering, electron-microscopy imaging, and nanoindentation experiments suggest that its mesoscale organization, extending over hundreds of nanometers, may be more important.

The new results gain insight into how the heterogeneities developed during the early stages of hydration persist in the structure of C–S–H and impact the mechanical performance of the hardened cement paste. Unraveling such links in cement hydrates can be groundbreaking and controlling them can be the key to smarter mix designs of cementitious materials.